Low profile mount for plural lower electrode metal oxide varistor package and method

ABSTRACT

A low profile mount for a disc varistor. A thermally sensitive switch is provided both for single and multiple electrode embodiments. The switch may be placed in a shorting circuit and include a spring biased conductor prevented from closure by a heat sensitive element which softens in responsive to excessive heat. The varistor may be fused to prevent excessive current from a short circuited but not open circuited varistor. Methods are also provided.

BACKGROUND OF THE INVENTION

[0001] This invention relates to a surface mount for an electricalcomponent and more particularly to a surface mount for a metal oxidevaristor.

[0002] Surface mounting of components on printed circuit boards iscommon and it is desirable that the mounts be low in profile so that thedistance between boards can be minimized and the size of the electronicchassis thus reduced. Where the electrical component is a thin elementsuch as a disc with electrodes on both flat sides, low profile mountsare particularly desirable.

[0003] It is also desirable that the number of components required formounting be minimized because of the area on the circuit board eachcomponent occupies. Thus it is highly desirable that the mount becompatible with components having multiple electrodes.

[0004] The thermal characteristics of the electrical components isgenerally a concern and it is desirable that the surface mount becompatible with, or integrated with one or more thermally sensitiveswitches. The thermal characteristics for many electrical circuitcomponents such as varistors are particularly important where thecomponent is coated with epoxy. In the event of a sustained overvoltagecondition across the component, the current through the component cancause the temperature to increase sufficiently to cause the epoxycoating to smoke or ignite. Similarly, the epoxy coating can smoke orignite in the event the component is subjected to a significant currentpulse such that the component internally shorts but does not destruct,i.e., does not open-circuit.

[0005] The current solution to this overheating problem is to wire inseries with the component a thermal cutout device in sufficiently closephysical proximity to respond to the heat in the component to shortcircuit and thereby disconnect the component. This solution requires theaddition of extra components and increases the cost and complexity ofthe circuit. Since great care must be taken to keep the thermal cutoutdevice in close physical proximity to the component to preserve theresponsiveness without overreacting, interference may occur with anysurface mount.

[0006] Additionally, the “thermal connection” between the component andthe thermal cutout device may be difficult to optimize in a surfacemount.

[0007] Finally, low temperature thermal cutout devices may not be ableto survive the soldering operation required to attach the component to aprinted circuit board or the like, and a surface mount to which thecomponent may be mounted without soldering is highly desirable.

[0008] Many of the above problems are exacerbated where the electricalcomponent is a disc varistor. Disc varistors are well known andgenerally comprise a thin disc of a metal oxide or other voltagevariable resistive material with an electrode on opposite flat sides ofthe varistor material. Known surface mounts for the smaller sizes, e.g.,7-10 mm, generally include a molded plastic body into which springelectrodes exiting the sides of the body are internally exposed inposition to be contacted by the electrodes of the disc varistor wheninserted therebetween from the front of the body.

[0009] One example of such prior art mount is illustrated in FIGS. 1 and2 where the disc varistor 18 comprises a disc of varistor material 20with surface electrodes 22 and 24 on the flat sides thereof. Thevaristor 18 may be inserted into the front opening of a plastic body 26which carries a top contact 28 and a bottom contact 30 convenientlyspring biased toward each other and separated by the insertion of thedisc varistor. The ends of the contacts 32, 34 may be used toelectrically connect the circuit to other components on the printedcircuit board on which the body 26 may be mounted. Such mounts areexpensive to manufacture and are generally higher in profile thandesired. Moreover, they do not provide for multiple electrodes, fortemperature protection or for a fuse.

[0010] Accordingly, it is an object of the present invention to obviatemany of the above problems and to provide a novel surface mount andmethod for an electrical component on a printed circuit board.

[0011] It is another object of the present invention to provide a novelsurface mount and method in which the mount is low profile.

[0012] It is still another object of the present invention to provide anovel surface mount and method in which the component may be connectedwithout soldering.

[0013] It is yet another object of the present invention to provide anovel surface mount and method which is readily adapted for multipleelectrical connections.

[0014] It is yet still another object of the present invention toprovide a novel surface mount and method which is compatible with athermal switch.

[0015] It is a further object of the present invention to provide anovel surface mount which is inexpensive and simple in construction.

[0016] It is yet a further object of the present invention to provide anovel surface mount and method which short circuit protection isavailable.

[0017] It is still a further object of the present invention to providea novel surface mount and method in which the mount may be coated with amoisture barrier and/or have edge passivation.

[0018] These and many other objects and advantages of the presentinvention will be readily apparent to one skilled in the art to whichthe invention pertains from a perusal of the claims, the appendeddrawings, and the following detailed description of the preferredembodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019]FIG. 1 is a pictorial view of a prior art surface mount packagefor a varistor disc.

[0020]FIG. 2 is a section in elevation taken through lines 2-2 of FIG.1.

[0021]FIG. 3 is a side view in elevation of one embodiment of thesurface mount of the present invention.

[0022]FIG. 3A is an exploded pictorial view of the mount of FIG. 3.

[0023]FIG. 4 is a side view in elevation of a second embodiment of thesurface mount of the present invention.

[0024]FIG. 5 is a side view in elevation of the embodiment of FIG. 4with a thermal switch in the open position.

[0025]FIG. 6 is a schematic circuit diagram of the varistor and switchof FIGS. 3 and 4 with a fuse in series with the varistor.

[0026]FIG. 7 a side view in elevation of a third embodiment of thesurface mount of the present invention adapted for two electrodes on theupper surface of the varistor.

[0027]FIG. 8 is a top plan view of a two component electrode compatiblewith the mount of FIG. 7.

[0028]FIG. 9 is a schematic circuit diagram of the varistor of FIG. 8.

[0029]FIG. 10 is a side view in elevation of a fourth embodiment of thesurface mount of the present invention adapted for two electrodes on thelower surface of the varistor.

DESCRIPTION OF PREFERRED EMBODIMENTS

[0030] With reference to FIGS. 3 and 3A, a preferred embodiment of themount of the present invention includes a molded plastic body 36 with acentral circular depression dimensioned to receive a disc varistor 18(varistor material 20 and electrodes 22 and 24) to a depth of aboutone-half the thickness thereof. Centrally located within the depressionis a second depression adapted to receive a circular contact 38 whichexits the body 36 to provide a first lead 40. The contact 38 desirablyextends upwardly above the bottom of the first depression in order toinsure contact with the bottom electrode 24 of the varistor 18.

[0031] The body 38 is desirably molded with a support 42 for the secondcontact 44 which may be made from an electrically conductive spring wireso that it is spring biased downwardly into contact with the upperelectrode 22 of the disc varistor 18. The contact extends through thesupport 42 to provide a second electrical lead 46 of the mount.

[0032] As shown in FIG. 4, the leads 40 and 46 may extend downwardly soas to penetrate through a printed circuit board for electricalconnection to an electrical circuit on the lower side thereof.

[0033] As described more fully in applicant's copending application Ser.No. filed concurrently herewith for “Thermally Protected Metal OxideVaristor And Method”, the disclosure of which is hereby incorporatedherein by reference, thermal protection may be provided by a thirdcontact 50 which may be constructed in the same manner as the contact 44to provide a third lead 52, but which includes a thermal element 54covering at least a portion of the contact 50 to prevent contact withthe upper electrode 22 of the varistor 18. The thermal element 54 may beany suitable conventional insulator responsive to excessive heat in thevaristor to either physically dissipate or to lose its insulativecharacteristics. An example of such a thermally reactive insulator isbenzanilide. Benzanilide is generally rigid up to its meltingtemperature, which is generally in the range of 150-200° C. and ispreferentially 163° C. Benzanilide adheres well and breaks down at itsmelting temperature to a viscosity that is water-like. It has been founddesirable for the body 36 to include a second support 56 at the end ofthe mount opposite to the support 42 to lend rigidity to the contact 50.

[0034] The circuit including the third contact 50 is shown in FIG. 6where a shorting circuit is shown with the leads 52 and 40 electricallyconnected. Under normal operating conditions, current flows through thevaristor 18 via the leads 40 and 46. An undesirable thermal condition inthe varistor 18 caused by a excessive current through the varistor froma sustained overvoltage condition across the varistor, or by a highcurrent pulse through the varistor which causes an internal short in thevaristor but does not cause the varistor to destruct, i.e., to opencircuit. In the event of such an undesirable thermal, the effectivedisappearance of the insulating means 54 in response thereto permitscontact between the electrode 50 and the upper electrode 22 of thevaristor and effects a short circuit across the varistor 18.

[0035] While the spring wire contact has been found desirable, it is tobe noted that other biasing means may be adequate, e.g., a discretespring for the electrical contact or a magnet. The shape of the varistoris not a part of this invention, and it may be square or other forms asdesired so long as the depression in the body of the mount iscompatible.

[0036] However, the mount of the present invention finds particularutility where the disc varistor electrodes are segmented as shown, e.g.,in FIG. 8. With reference to FIG. 8, one of the electrodes, e.g.,theupper electrode 22, may be segmented into two, three or four parts, eachforming a varistor element with the single lower electrode 24.

[0037] As shown in FIG. 7, an embodiment with two segments on the upperelectrode may use two spring wire contacts 60, 62 connected respectivelyto leads 64, 66 to form the circuit shown in FIG. 9. As shown in FIG.10, the identical circuit may be formed using the two segment varistorof FIG. 8 with the segmented electrode facing downwardly and a singlecontact 44 in contact with the single, now upper, electrode 24. Thisembodiment requires that the body 36 include not one central contact 38as illustrated in FIG. 3A, but two spaced contacts 38A and 38B as shownin FIG. 10.

[0038] While preferred embodiments of the present invention have beendescribed, it is to be understood that the embodiments described areillustrative only and that the scope of the invention is to be definedsolely by the appended claims when accorded a full range of equivalence,many variations and modifications naturally occurring to those of skillin the art from a perusal hereof.

What is claimed is:
 1. A low profile mount for a disc varistorcomprising: an insulative base having a circular receptacle for avaristor disc; an exposed bottom electrical contact in said receptaclein position to contact an electrode on the bottom flat side of avaristor disc when placed in said receptacle; and an upper electrodeextending upwardly from an said base outside of said receptacle andbeing downwardly biased toward said base over said receptacle inposition to contact the electrode on the bottom flat side of a varistordisc when placed in said receptacle.
 2. The mount of claim 1 whereinsaid upper electrode is a conductive metal spring.
 3. The mount of claim1 wherein the low profile device is surface mounted on a printed circuitboard.
 4. The mount of claim 1 wherein the low profile device is throughhole mounted on a printed circuit board.
 5. The mount of claim 1 whereinthe disc varistor is 14-20 mm in diameter.
 6. The mount of claim 1wherein the disc varistor is 7-45 mm in diameter.
 7. The mount of claim1 wherein the low profile device is coated with a moisture barrier. 8.The mount of claim 1 including a shorting electrode upwardly extendingfrom said base outside of said receptacle and being downwardly biasedtowards said base over said receptacle in position to contact theelectrode on the upper flat side of a varistor disc when placed in saidreceptacle, said shorting electrode being electrically connected to saidbottom electrode; and an insulator intermediate said shorting electrodeand the upper electrode of a varistor disc when placed in saidreceptacle in position to prevent contact between the shorting electrodeand the upper electrode, said insulator being thermally reactive topermit contact between said shorting electrode and the upper electrodeof a varistor disc under the bias of said shorting electrode whereby theelevation in the temperature of a varistor disc in said receptaclecauses said insulator to react to short the electrodes of the varistordisc.
 9. The mount of claim 8 wherein said shorting disc is a metalspring.
 10. The mount of claim 8 wherein said shorting disc ismagnetically biased.
 11. The mount of claim 8 wherein said insulatormelts at temperatures above a predetermined value.
 12. The mount ofclaim 8 wherein the insulator deteriorates between 150-200° C.
 13. Themount of claim 8 wherein the insulator deteriorates at 163° C.
 14. Themount of claim 8 wherein the insulator consists essentially ofbenzanilide.
 15. The mount of claim 1 including at least one additionalupper electrode extending upwardly from an said base outside of saidreceptacle and being downwardly biased toward said base over saidreceptacle in position to contact an electrode on the top flat side of avaristor disc when placed in said receptacle, said upper electrodesbeing spaced apart to thereby be positioned to contact different upperelectrodes on a segmented varistor disc.
 16. The mount of claim 1including at least one additional exposed bottom electrical contact insaid receptacle in position to contact an electrode on the bottom flatside of a segmented varistor disc when placed in said receptacle.
 17. Amount for an electrical component having exposed upper and lowerelectrodes comprising: a insulative body with an upwardly openreceptacle adapted to receive an electrical component; a lower contactcarried by said body within said receptacle in position to contact thelower electrode of a component when received therein: an upper contactcarried by said body above said receptacle in position to contact theupper electrode of the component when received therein, said uppercontact being downwardly biased toward said receptacle.
 18. The mount ofclaim 17 including a second upper contact carried by said body abovesaid receptacle in position to contact a second upper electrode of thecomponent when received therein, said second upper contact beingdownwardly biased toward said receptacle.
 19. The mount of claim 18wherein said second upper contact is electrically connected to saidlower electrical contact; and including a thermally responsive insulatorbetween said second upper contact and the upper electrode of theelectrical component for preventing contact therebetween underpredetermined thermal conditions within the component, said insulatorbeing responsive to a predetermined thermal condition within thecomponent to permit contact between said second contact and the upperelectrode of the component.
 20. The mount of claim 19 wherein theinsulator consists essentially of benzanilide and wherein the insulatordeteriorates between 150-200° C.
 21. The mount of claim 19 wherein theinsulator deteriorates at 163° C.
 22. The mount of claim 18 wherein saidsecond upper contact is electrically isolated from both said lowercontact and said first mentioned upper contact to thereby adapt saidmount for use with components with segmented upper electrodes.
 23. Amount for an electrical component having an exposed lower electrode anda plurality of exposed upper electrodes comprising: a insulative bodywith an upwardly open receptacle adapted to receive an electricalcomponent; a lower contact carried by said body within said receptaclein position to contact the lower electrode of a component when receivedtherein: a plurality of spaced upper contacts carried by said body abovesaid receptacle in position to contact spaced upper electrodes of thecomponent when received therein, each of said plurality of uppercontacts being downwardly biased toward said receptacle.
 24. The mountof claim 23 wherein said plurality is four.
 25. The mount of claim 24wherein said component is a varistor.
 26. A mount for an electricalcomponent having an exposed upper electrode and a plurality of exposedlower electrodes comprising: a insulative body with an upwardly openreceptacle adapted to receive an electrical component; a plurality ofspaced lower contacts carried by said body within said receptacle, eachof said lower contacts being in position to contact one of the segmentedlower electrodes of a component when received therein: an upper contactscarried by said body above said receptacle in position to contact theupper electrodes of the component when received therein, said uppercontacts being downwardly biased toward said receptacle.
 27. The mountof claim 26 wherein said component is a varistor.
 28. The mount of claim27 wherein said plurality is four.
 29. A method of mounting anelectrical component having exposed upper and lower electrodes on aprinted circuit board comprising the steps of: (a) providing aninsulative body having an upwardly open receptacle adapted to receive anelectrical component; (b) positioning a lower contact within thereceptacle in the body in position to contact the lower electrode of acomponent when received therein: (c) providing an upper contact carriedby the body above the receptacle in the body in position to contact theupper electrode of the component when received therein; (d) downwardlybiased the upper contact toward the receptacle in the body, so that anelectrical component when received within the receptacle in the bodywill electrically contact the lower contact within the receptacle and becontacted by the upper contact under the downward bias thereof.
 30. Themethod of claim 29 wherein the component is a varistor.